IHRA Chevy Big Block - From Street To Strip

Prepping The ZZ 502 Crate Motor For IHRA Top Stock Competition

Unlike most connecting rod caps, which are torqued to 80 ft-lb, Carillo says its new, high-strength bolts must be twisted to 100 ft-lbs.

Kit In A CrateThe Top Stock rules demand a crate engine package, which Bogan's ZZ 502 certainly is, but McLaren Engines started with an unassembled 502 crate motor kit (the 502 is available from GM in kit, long-block or fully assembled versions), PN 12371171. Because many components, such as the pistons and rods, would be replaced, starting with the kit saved time that would have otherwise been spent disassembling the long-block. The photos that accompany this story provide a thorough description of what it takes build the street-friendly Chevy 502 into a competitive race engine.

In a nutshell, the engine will spend most of its time between 5,000 and 7,000 rpm, and it's built to "live" there. Like almost every other race engine, this 502 draws its breath through a single-plane intake manifold, which helps the high-rpm velocity. Interestingly, IHRA is pretty tight about the carburetor, requiring it to have a choke. So, an out-of-the-box Holley 850 Double Pumper mixes the air and fuel.

A big difference between this race engine and a street engine is the rod side clearance. Striking a balance between adequate oil flow, but not at the expense of consumption or windage, the clearance between the rods of this Top Stock big-block averaged about .025 inch, which is considerably looser than a street motor.

Rules set down by IHRA for Top Stock competition are designed to keep racers within reasonable budgets. This is why, for example, the choke is mandatory for the carburetor; it dissuades the owner from spending tons of money on a completely custom carb. Same goes for the off-the-shelf intake manifold and cylinder heads.

For engine builder Halvorson, such restrictions aren't a hindrance, they're just the rules of the game, and he uses them to define his game plan. Though he can't port the heads, gaining high-rpm flow is still a goal which can be attained by using a custom cam (as long as it retains the stock specs).

But since the rules stipulate things like stock camshaft lift, stock combustion chamber volume, and stock piston design, all of the obvious horsepower-increasing tricks seem out of reach. For example, a three-angle valve job is allowed, but angle-milling the heads and any port work is strictly off limits. So, Halvorson turned to the engine's finer details. By swapping the street-based, dual-plane intake manifold for a high-velocity single-plane part, he was able to feed the engine at high rpm and, correspondingly, spec a cam with lots of duration. At the upper end of the rpm range, this combination really makes the big-block come alive.

But Halvorson is quick to point out the power gains don't come from just one place and says that several horses were picked up by swapping the engine-driven water pump with an electric pump. Also, great gains were realized by designing an oil pan that virtually eliminates crankcase windage. These are parts that wouldn't work on a street engine, but they're race-legal and give the engine a competitive edge.

Indeed, on the dyno, this engine produced 657 hp-about 30 percent better than stock-with a stock-lift cam, stock-style pistons, and virtually the stock compression (9.84:1 vs. the stock rating of 9.6:1). All that, and an off-the-shelf, four-barrel carb, too.

Other changes, such as the connecting rods and the addition of a camshaft thrust bearing, were done more for racing durability and high-rpm accuracy than horsepower gains. Most surprising, though, is the number of parts not changed in the build-up. Halvorson credits the crate motor's inherent strengths. "We could have changed the crank but didn't have to," he says. "The GM crank from the kit is already a forged, balanced part that's as strong as anything needed for Top Stock."

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IHRA Chevy Big Block - From Street To Strip

IHRA rules mandate a manufacturer's crate engine. In the case of the Chevy 502, it's available in kit form, which means a bare block to begin the assembly. Although many components of the street-intended kit are replaced for this racing application, a surprising number of them are retained. McLaren overbored the engine .060 inch, even though IHRA allows a .070-inch overbore. The .060 job leaves room for a future rebuild.

Before any of the reciprocating assembly was installed, the engine was treated to a thorough cleaning and deburring of casting flash and other sharp edges. Here, the oil galley at the rear main bearing cap was smoothed and radiused. This minor step in block preparation smoothes the transition of oil out of the galley, which reduces cavitation and oil pressure fluctuation. And since the engine will likely be run consistently with less than 3 quarts of oil, ensuring constant oil pressure is an obvious concern.

There are four oil-return ducts in the big-block's lifter valley. For the Top Stock engine, the front three are blocked off, while the rearmost duct is screened off, and a small trough below directs returning oil to the back of the engine. This is done to prevent the oil from dripping onto the crankshaft, thereby causing windage. The blocking plates and trough were handmade from .040-inch aluminum, while the plates and screen were affixed with Devcon epoxy.

As the stock crank in the 502 crate engine package is a balanced, forged piece, it's retained for the race engine. But, it was rebalanced after the engine's new pistons and rods were fitted to it. The existing external balance holes were simply drilled farther-McLaren took 105 grams from the front of the crank and 115 grams from the rear. Once rebalanced, the crank was micro-polished before being dropped in the block. Installed, crank endplay measured .005 inch, which is close to a street engine's specs. If the engine were to be used in a manual-transmission car, more clearance, from .007-.010-inch, would be acceptable, as the rear of the crank would typically endure more heat expansion.

The stock main bearing caps are used, and ARP studs are used for added strength and less wear from constant removal. The caps can be torqued down a bit more accurately with the studs, too. In this case, the caps are torqued to 100 ft-lb in a two-step process.

Because the engine will be used strictly in a high-rpm racing environment, McLaren swapped the 502's stock, but nonetheless stout, rods and pistons for stronger, race-specific parts (stock rod and piston are in the background). A piston with a full floating wrist pin was desired (stock is forged with a pressed pin) because there's less friction load at higher rpm and the piston is less likely to be damaged if the engine is disassembled. The new pistons are from Ross and are the only IHRA-approved slugs for Top Stock. Per the rules, the wrist pin location is stock, but the location of the "ring pack" is a variable. As for the rods, they're lightweight, forged H-beams from Carillo, with mandatory stock (6.135-inch) lengths. Interestingly, there's no specific compression ratio limit in the rules, but with cylinder head restrictions, the new pistons will provide compression in the 9.8:1 area.

Unlike most connecting rod caps, which are torqued to 80 ft-lb, Carillo says its new, high-strength bolts must be twisted to 100 ft-lbs.

A big difference between this race engine and a street engine is the rod side clearance. Striking a balance between adequate oil flow, but not at the expense of consumption or windage, the clearance between the rods of this Top Stock big-block averaged about .025 inch, which is considerably looser than a street motor.

For Top Stock, the camshaft can be replaced as long as it retains stock lift specs. There's no restriction on duration, however. To that end, a custom-grind from Comp Cams was ordered. With lift specs at .530 on the intake side and .542 on the exhaust, the cam's duration measures 248 on the intake valves and 254 on the exhaust. This spread results in a 110-degree lobe separation.

To limit cam "walking" at high rpm, a Torrington thrust bearing is added to the timing gear. The bearing, which is placed behind the timing gear, will still allow limited movement of the camshaft-about .005 to .008 inch. A cam button is fitted, too, and it also features an internal Torrington bearing. Adding the thrust bearing isn't a necessary step, even for a race engine, but McLaren added it for the sake of timing accuracy.

Two things to note here: First, the Weiand racing electric water pump was added because it runs at full capacity whenever it's on: a quality not necessary for a street engine. Also, since it's electric, it doesn't require any engine power to drive it with an accessory belt. Second, the ATI Super Balancer damper is smaller and lighter than stock, but it's allowed because IHRA demands an SFI-approved balancer for safety.

Stock heads must be retained for Top Stock, which is just fine for this engine, as the 502's oval-port 356-T6 aluminum heads boast 2.25-inch intake valves and 1.88-inch exhaust valves. The springs can be changed as long as the replacements are the stock diameter. In this case, a heavy-duty set of dampening springs was added to stand up to the more aggressive camshaft and the engine's high-rpm lifestyle.

No porting is allowed, but a three-angle valve job is okay. In this case, it was performed on McLaren's state-of-the-art Serdi 2000 "valve seat and guide machining center." The heads were also flow-tested to help determine the camshaft specs.

The rules stipulate the heads can't be ported. The 502's oval-port heads (PN 12363399) have good-sized 290cc intake runners and 110cc exhaust ports, along with 2.25/1.88 valves. With the stock-type, small-top pistons, this motor's compression ratio is 9.84:1. That's great for the street, but a race engine could always use more. Unfortunately, the rules forbid reducing the combustion chamber volume, which would increase the ratio.

IHRA says combustion chambers must remain stock, which, in the case of the Chevy 502, is 110cc. So, McLaren cc'd the chambers and discovered some slight inconsistencies between them. The heads were resurfaced slightly to true-up the chambers to ensure they were all at 110cc (most were slightly larger).

Engine builder Curtis Halverson carefully drops the cylinder heads in place. Like the main bearings, the cylinder heads are installed over studs. Again, the idea is to limit the amount of wear and tear on the block if the heads need to be removed.

The lifter bores are drilled out and fitted with stronger brass inserts. The inserts are indexed, too, to ensure alignment with the cam's lobes. Unlike a small-block, the big-block's lifters aren't all on the same plane, so lining them up correctly will reduce friction. Also, the inserts are drilled to match the oil flow holes in the block.

Although the lifters are stock, the stock, stamped steel rocker arms are replaced with GM's (PN 12361323) 1.7:1 roller rockers. The bodies of these rockers are aluminum, but the pushrod seats are 8620 steel.

The 502 engine package comes with a dual-plane intake manifold. For racing it is replaced with a single-plane Edelbrock Victor manifold that matches the heads' oval-port design. Check out the valve covers, too. They're needed to clear the big, 1.7-ratio roller rockers. The stock valve covers are too low.

To limit the cost of competition in Top Stock, IHRA mandates that each carburetor has a choke. This effectively limits carburetor selection to off-the-shelf stuff (with some minor tweaks or blueprinting). For this engine, a Holley 850 Double Pumper was used. On the dyno, it performed well with the factory-fit #68 jets. The floats were simply set up, and the engine starting making full-throttle pulls. Also, the 1-inch phenolic spacer is IHRA-legal.

Though a seemingly simple part, the oil pan of this Top Stock engine represented dozens of hours in design and construction. It was designed specifically for the chassis of Monty Bogan, Jr.'s '69 Camaro. Within those dimensions, however, the pan is shaped to eliminate crankshaft windage, and, knowing how little oil will likely be used, supply every possible drop of oil to the pump pick-up. Although a wet-sump oiling system, the final design was influenced by the shape of NASCAR-style dry-sump pans.

Like the oil pan, this engine's headers involved hours of careful fabrication (by Paul Swindelhurst of McLaren's fabrication shop). The tuned, equal-length design was based both on the headers' fit in the car and the approximate horsepower of the engine. Even the cylinder heads' flow was factored into the design and tubing diameter. Also, they were designed for easy spark plug access. The 23-inch-long headers boast 2-inch primaries and 3.5-inch merge collectors.

Strapped to one of McLaren's Froude water-brake dynos, the Top Stock big-block was tested between 2,500 rpm and 7,250 (ignition system is MSD throughout). The engine was run with AC R42XLS spark plugs, Mobil 1 10W30 oil, and this timing curve: 15 degrees at 1,000 rpm, 20 at 1,500, 24 at 2,000, and 36 degrees from 2,500 to 7,500 rpm. With less than 10:1 compression, this big-block makes more than 430 lb-ft at 2,500 rpm. From there, it's a smooth, gradual progression to its peak at 5,300 rpm, where it's twisting out 597 lb-ft. As for horsepower, it's a sharper rise, from just more than 300 horses at 3,000 rpm to 657 horses at 6,100 rpm. In all it's a relatively simple, but carefully thought-out, combination that should push Monty Bogan, Jr.'s yellow Camaro into the winner's circle.

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